Magnetrons

Want to cook a dinner in five minutes or make an airplane safer to
fly in bad weather? You'll be needing some microwaves, then. Those are the invisible,
super-energetic, short-wavelength radio waves that travel at the
speed of light, doing the important stuff in microwave ovens and
radar-navigation equipment.
Making microwaves is easy if you have the right
equipment—a handy gadget called a magnetron. What is it and
how does it work? Let's take a
closer look!

Photo: The CV64 cavity magnetron, developed in Birmingham in 1942, was small enough to fit inside an airplane. Devices like this made it possible for planes to use radar defenses for the first time. An exhibit at Think Tank (the science museum in Birmingham, England). Sorry about the slightly poor quality of the image: the exhibit is inside a glass case and hard to photograph.

How does a magnetron work?

Magnetrons are horribly complicated. No, really—they're horribly
complicated! To understand how they work, I find it helps to compare
them to two other things that work in similar ways: an old-style TV
set and a flute.

A magnetron has quite a lot in common with a cathode-ray
(electron) tube, the sealed glass bulb that makes the picture in an
old-style television set. The tube is the heart of a TV: it makes the
picture you can see by firing beams of electrons at a screen covered
in chemicals called phosphors so they glow and give off dots
of light. You can read all about that in our main article on
television, but here (briefly) is what's happening. Inside the TV,
there's a negatively charged electrical terminal called a cathode
that's heated to a high temperature so electrons "boil" off it.
They accelerate down the glass tube, attracted by a positively
charged terminal or anode and reach such high speeds that they
race past and crash into the phosphor screen at the tube's end. But a
magnetron doesn't have the same purpose in life as a TV. Instead of making a
picture, it's designed to generate microwaves—and it does that a little bit
like a flute. A flute is an open pipe filled with air. Blow across
the top in just the right way and you make it vibrate at a specific
musical pitch (called its resonant frequency), generating a
sound you can hear that corresponds directly to the length of the
pipe.

A magnetron's job is to generate fairly short radio waves.
If you could see them, you could easily measure them with a school ruler.
They're usually no shorter than about 1mm (0.04 in; the shortest
division on a metric ruler) and no longer than about 30cm (12in; the
length of a typical school ruler). The magnetron does its stuff by
resonating like a flute when you pump electrical energy into it. But, unlike a
flute, it produces electromagnetic waves instead of sound waves so
you can't hear the resonant energy its making.
(You can't see that energy either, because your eyes aren't sensitive to short-wavelength, microwave
radiation).

Artwork: Right: One of the drawings of the high-energy magnetron developed in the 1940s by Percy Spencer, who went on to perfect the microwave oven while working at Raytheon. (I've colored it in to match my own artwork below.) You can see a bigger version of this drawing and read the full technical details via Google Patents.
Artwork courtesy of US Patent and Trademark Office.

If you switched on a simple magnetron like this, electrons would boil off from the
cathode and zip across to the anode in straight lines (shown by the black arrow) much like the
electron beam in a TV set. But there are two added extra bits in a
magnetron that change things completely.

First, the anode has holes or slots cut into it called cavities or resonant cavities.
Second, a powerful magnet is placed underneath the anode to generate
a magnetic field along the length of the tube (parallel to the
cathode and, in this diagram, going directly into the computer screen away from you).

Now when the electrons try to zip from cathode to anode,
they are traveling through an electric field (stretching between the
anode and cathode) and a magnetic field (produced by the magnet) at
the same time. So, like any electrically charged particles moving in a magnetic field,
they feel a force and follow a curved path (blue circle) instead of
a straight one, whizzing around the space between the anode and the
cathode.

As the electrons nip past the cavities, the cavities
resonate and emit microwave radiation. Think of the electrons passing
energy to the cavities, making then resonate like someone blowing
on the open end of a flute—only producing microwaves instead of sound waves.

The microwave radiation that the cavities produce is
collected up and channeled by a kind of funnel called a waveguide,
either into the cooking compartment of a microwave oven or beamed out
into the air by an antenna or satellite dish in radar equipment.

In reality, it's all a bit more complicated than that—of course. But think of
a TV set and a flute sort of merged together to produce microwaves instead of
flute sounds or TV pictures and you'll get the basic idea!

A brief history of magnetrons

1920s: American engineer Albert W. Hull invents the first magnetron while working for General Electric.

~1940: Two physicists, John Randall and Harry Boot, working at
the University of Birmingham, England develop a much more powerful
magnetron that is compact enough to fit into ships, planes, and
submarines.

1940s: American engineer Percy Spencer accidentally discovers
that microwaves produced by a magnetron have enough power to heat
and cook food. He patents the microwave oven in the 1950s.

Photo: There's a magnetron tucked inside your microwave oven, usually just behind the control and instrument panel on the right. If you open the door, you can sometimes get a glimpse of the magnetron and its cooling fins through the perforated metal cage that separates it from the main cooking compartment.

Articles

Patents

If you want to read detailed technical descriptions of how magnetrons are designed and how they work, patents are an excellent place to start. They're not always that easy to understand, but the descriptions are extremely detailed and there are generally very clear labeled diagrams. Here are a few to start you off; you'll find lots more if you search at the USPTO (or Google Patents) using the keyword "magnetron":